Capture of volatilized vanadium and tungsten compounds in a selective catalytic reduction system

ABSTRACT

An apparatus and method for treating diesel exhaust gases are described. The system consists of two functionalities, the first being a selective catalytic reduction (SCR) catalyst system and the second being a capture material for capturing catalyst components such as vanadia that have appreciable volatility under extreme exposure conditions. The SCR catalyst component is typically based on a majority phase of titania, with added minority-phase catalyst components comprising of one or more of the oxides of vanadium, silicon, tungsten, molybdenum, iron, cerium, phosphorous, copper and/or manganese vanadia. The capture material typically comprises a majority phase of high surface area oxides such as silica-stabilized titania, alumina, or stabilized alumina, for example.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. Ser. No. 12/638,166,filed Dec. 15, 2009, the entire contents of which is hereby expresslyincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The selective catalytic reduction (SCR) of nitrogen oxides produced incombustion engines, with reductants such as urea and ammonia, is anindustrially significant catalytic process. Vanadia-based SCR catalyststhat utilize titania catalyst supports are approved for use for on-roadmobile applications in Europe on Heavy-Duty Diesel trucks, and thesecatalyst are highly active and show excellent tolerance to fuels thatcontain sulfur. However, vanadia-based catalysts are not approved by theEPA for on-road use in the U.S. or in Japan. This lack of approval stemsfrom the concern over release of vanadia into the environment and thepotential toxicity that might arise from exposure to vanadia emittedfrom the tailpipe. One possible mechanism that potentially might cause aloss of vanadia from the catalyst is vaporization of the metal oxide orhydroxide at high temperature in the stream of hot exhaust gases.

Furthermore, newer regulations for soot and NOx that will begin to beimposed as early as 2010 (e.g., Euro VI and US 2010 regulations) maynecessitate the use of both a diesel particulate filter (DPF) in tandemwith an SCR catalyst. In one configuration (U.S. Pat. No. 7,498,010) theSCR catalyst is downstream from the DPF. If no remedial action is taken,the collection of soot on the DPF, will ultimately plug the channels forexhaust gas flow, and can cause unacceptable pressure-drop arises acrossthe device. In order to avoid this situation, the soot is removed eithercontinuously or sporadically by combustion. Since combustion is anexothermic process, it is associated with a rise in the temperature ofthe device that is transmitted to the exhaust gases, and the temperaturerise depends on the amount of soot collected as well as the temperatureof the exhaust gas upstream of the DPF. These high temperature exhaustgases, which can approach 750° C. and higher, subsequently will passover the SCR catalyst. Thus, there has been much emphasis recently onimproving the thermal stability of the SCR catalyst, both forvanadia-based catalysts as well as Cu—, Fe— and other base-metalcatalysts. It is generally accepted that the catalyst must be stable totemperatures of up to 800° C. for short periods of time. In order totest the durability of catalyst formulations, it is necessary to developtests that simulate real-world exposure conditions. Ford researchers[1]have developed an accelerated aging protocol for SCR catalysts thatsimulates on-road conditions over a span of 120,000 miles. This testinvolves exposing the catalyst to a reactant gas stream that includeswater (5 vol %), for a time of 64 hr at 670° C. at relatively high gasflow rate (gas hourly space velocity, GHSV=30,000 hr⁻¹). Theseconditions of time and temperature are used as a reference point for themethod of the present invention.

Concern over the volatility of vanadia at high temperatures, e.g., whenthe SCR catalyst is located downstream of the DPF, is thus an issue thatmay limit the available market for vanadia-based mobile SCR catalystsand is a key consideration in catalyst development. There has thusremained in the art a need to be able to evaluate the degree of vanadiavolatilization from SCR catalysts. Further, there has remained a need inthe art for a deNOx selective catalytic reduction catalyst system whichdemonstrates zero vanadia loss downstream thereof. It is an object ofthe invention to address these shortcomings of the prior art.

SUMMARY OF THE DISCLOSURE

The present disclosure describes compositions and processes for thecapture of volatilized vanadium and tungsten compounds in a selectivecatalytic reduction catalyst system, for example in an emission controlsystem of a diesel engine.

In one aspect of the disclosure, it is contemplated that stable, highsurface area oxide supports, including but not limited to,silica-stabilized titania and alumina, can be employed to capture thesevolatile components, when said components are present at low density onthe support surface of the capture bed after their capture. Further, inregard to certain embodiments, it has also been surprisingly found thatwhen the normally volatile components are present at low density on thecatalyst surface prior to exposure to extreme conditions, they alsoexhibit diminished volatility at the extreme conditions, so that such“low density” catalysts can also be employed to reduce or eliminatecatalyst component volatility. Thus, the volatile components, oncecaptured, preferably will not be substantially released again into theexhaust phase. Therefore, a key aspect of this invention is to providefor high stability, high surface area inorganic oxide supports that canbe used in a configuration wherein the stable, high surface areasupports are either mixed with, or are positioned downstream from alower stability, V-based SCR catalyst material in which the catalystcomponents are present at relatively high surface density, wherein theformer stable, high surface area supports capture and remove volatilecompounds from the vapor phase even at the same temperature which ispresent in the lower stability and higher surface density catalystmaterial from which the compounds were volatilized.

The present summary is not intended to be an exhaustive or completesummary of the disclosure but is only intended to identify variousnotable aspects thereof. Other aspects of the disclosure notspecifically noted above will become evident upon consideration of thedescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of a selectivecatalytic reduction catalyst system of the invention wherein thecatalyst material is positioned upstream of the capture material.

FIG. 2 is a schematic representation of another embodiment of aselective catalytic reduction catalyst system of the invention whereinthe catalyst material and capture material are combined in a mixture.

FIG. 3 is a graph showing the relationship between the Total FractionalMonolayer values and the vanadium volatility

FIG. 4 is a graph showing the relationship between the Total FractionalMonolayer values and the tungsten volatility.

DETAILED DESCRIPTION OF THE INVENTION

In the course of investigation of the vaporization of catalystcomponents (supported on oxide supports) during high-temperature,accelerated aging tests, it has surprisingly been found that thecatalyst components exhibit varying degrees of volatility depending onthe catalyst support, and the volatilities of the supported catalystmaterials can be substantially different from volatilities of the bulkoxides. In the event that the supported catalyst shows non-negligiblevaporization of the catalyst components, it is desirable to provide ameans of capturing the volatilized components released into the vaporstream. One means of capturing such components that are volatile at hightemperature is simply to allow them to condense at a lower temperaturedownstream of the catalyst bed. However, this approach is problematicbecause the volatile components may condense on unintended locations.Thus, it is even more desirable to provide a means for capturing thevolatile components even at very high sustained temperatures such asmight be encountered in the catalyst bed. The present inventiontherefore is directed to a treatment system for diesel engine exhaust,and use therewith, which comprises a vanadia-based selective catalyticreduction catalyst (V-based SCR catalyst) system and which comprises a“capture-bed” either mixed with, or immediately downstream of theV-based SCR catalyst. The function of the capture bed is to capture andretain any volatile compounds even at elevated temperatures and gas flowconditions which approach those experienced in the catalytic portion ofthe catalyst/capture bed mixture, or in the upstream catalyst bed, sothat the volatile components are thus removed from the vapor phase ofthe exhaust gas.

In one embodiment of the invention, where the catalyst material andcapture bed material comprise a mixture, the v/v ratio of the catalystmaterial and capture bed material may be, for example, in the range of1:20 to 20:1, and more preferably be, for example, 1:10 to 10:1.

Further, where used herein, the terms “catalyst bed,” “catalystmaterial,” and “catalyst bed material” may be used interchangeably.Similarly, the terms “capture bed, “capture material,” and “capture bedmaterial” may be used interchangeably.

Where used herein the term “substantially all” means at least 90% of thematerial which is referred to, or more preferably to at least 95% of thematerial which is referred to, or more preferably to at least 96% of thematerial which is referred to, or more preferably to at least 97% of thematerial which is referred to, or more preferably to at least 98% of thematerial which is referred to, or more preferably to at least 99% of thematerial which is referred to.

It has presently been found that stable, high surface area oxidesupports, including but not limited to, silica-stabilized titania oralumina, can be employed to capture the volatile components, when saidcomponents are present at low density on the support surface of thecapture bed material after their capture. Further, in regard to certainembodiments, it has also been surprisingly found that when the normallyvolatile components are present at low density on the catalyst surfaceprior to exposure to extreme conditions, they also exhibit diminishedvolatility at the extreme conditions, so that such “low density”catalysts can also be employed to reduce or eliminate catalyst componentvolatility. Thus, a key aspect of this invention is to provide for highstability, high surface area inorganic oxide supports that can be usedin a configuration wherein the stable, high surface area supports areeither mixed with, or are positioned downstream from a lower stability,V-based SCR catalyst material in which the catalyst components arepresent at relatively high surface density, wherein the former stable,high surface area supports capture and remove volatile compounds fromthe vapor phase even at the same temperature which is present in thelower stability and higher surface density catalyst material from whichthe compounds were volatilized.

This configuration, shown in a schematic diagram in FIG. 1, can beachieved, in one exemplary embodiment by preparing wash-coated catalyststhat are “zoned”, that is, the lower stability, higher surface densityV-based SCR catalyst is located in a position in front of the device,while the high stability, high surface area low surface densityinorganic oxide is located in a position that is towards the back of thedevice.

Another configuration, shown in a schematic diagram in FIG. 2, can beachieved in one exemplary embodiment by the co-extrusion (or other modeof mixing) of catalyst materials and capture bed materials. When thehigh stability, high surface area oxide is comprised of a compositionthat is a good support for V-based SCR catalysts, then the capturematerial may be an active catalyst even as it accumulates the volatileoxides from the less stable catalyst. Titania-based materials that aresuitable to be used in this configuration as the highly stable, highsurface area supports are those described in a recent patent application(U.S. Ser. No. 12/533,414). It will be seen below that an example of alower stability, higher surface density V-based SCR catalyst is 2 wt %vanadia supported on DT-52. This catalyst is nonetheless a highly activeSCR catalyst under normal conditions, but exhibits considerable loss ofsurface area after exposure to extreme conditions. Thus, one embodimentfor the present invention comprises a mixture of a vanadia on DT-52catalyst material with the titania-based materials or catalystsdescribed in the recent application (U.S. Ser. No. 12/533,414). Forexample, the mixture can be achieved either in wash-coat or extrusionapplications or any other suitable method for creating highlyinterspersed mixtures of particulate materials. In a second embodiment,the present invention comprises a system of a high surface area, highstability alumina (or any other suitable capture bed material)positioned downstream of the vanadia on DT-52 catalyst (or any otherappropriate catalytic material).

The present invention will thus enable the use of V-based SCR catalystsin configurations where the catalyst material and also the capture bedmaterial, will be exposed to very high temperatures.

It is helpful to consider parameters that serve to define the capturematerials, and one such parameter involves the surface density ρ_(surf),(atoms/nm²) of catalyst components on a support material. The catalystand capture materials of the present invention are typically composed ofa majority phase (mass fraction greater than about 0.7) and one or moreminority phases (mass fraction less than about 0.3) wherein the minorityphase is viewed as existing on the surface of the majority phase. Forexample, titania-based SCR catalysts typically contain a minority phasecomprising vanadia (generally less than 5%), tungsta (generally lessthan 15%), and optionally silica (generally less than 15%), that havebeen deposited on the surface of the titania (majority phase). When theminority phases are present at very low mass fraction, they can exist inan atomically dispersed state, bonded solely to the support material. Bycontrast, when the minority phases are present at higher mass fractions,they can begin to form bonds homogeneously and thus form one or morelayers on the support material. In extreme cases, the minority phasesmay actually crystallize, forming e.g., bulk crystalline tungsta in itsnative monoclinic crystalline form, in intimate mixture with the supporttitania. In this state, it is possible that the minority phases developchemical character more typical of that of the bulk phase of theminority oxide. In the particular cases of vanadia and tungsta, the bulkoxides may exhibit volatility under extreme conditions of temperature,water vapor, gas flow rate and time.

As can be ascertained from the chemical literature, the maximum amountof vanadia that can be maintained in a highly dispersed state on titaniawithout vanadia crystal formation[2] is 7.1 V atoms/nm², and thisdensity is assumed to represent monolayer coverage. Monolayer coverageof highly dispersed tungsta on titania[3] is estimated to occur at atungsta loading of 4.5 W/nm², and that for silica[4] is estimated tooccur at a silica loading of 6 Si atoms/nm². These complete, ormonolayer surface densities, ρ_(surf, monolayer), for the highlydispersed minority phases can be used to further define the actualfractional coverage at the mass fraction, f_(m, i), of the minorityphases (i) in the real catalyst:f _(m,i)=ρ_(surf,i)/ρ_(surf,monolayer,i)

In the case that there are multiple supported oxides (e.g., silica,tungsta and vanadia) in the real catalyst, the total fractionalmonolayer coverage Tf_(m) can then be defined as the sum of the f_(m,i)for each of the minority-phase supported oxides. From the abovedefinitions, it is apparent that the combination of high majority-phasesupport surface area and low minority-phase mass fraction results invery low fractional coverage for the component oxides. This combinationis very desirable for the capture materials of the present invention.Furthermore, since the object of the present invention is to capturevolatile oxides under conditions of extreme exposure, it follows that itis highly desirable that the surface area for the majority-phase oxidebe stable and not substantially decrease as a result of the exposureconditions. The relevant surface area to be used in this definition isthat which is measured after exposure of the capture or catalystmaterials to the severe conditions that simulate lifetime exposure.Stable surface area thus means that there is minimal loss in surfacearea from the fresh state (before exposure) and the aged state (afterexposure). The condition of low fractional coverage is also a desirablefeature of the catalyst materials themselves, as the componentvolatility is surprisingly found to be minimal under that condition. Itfollows that it is also desirable for the catalyst material to retainappreciable surface area during exposure to harsh conditions.

The examples and embodiments herein and below refer to various titaniaand alumina materials which may comprise the majority phase of thecatalyst support material or capture bed material of the invention.However, the majority phases of the catalyst support and capture bedmaterials are not to be limited to these and may also comprise, alone orin combination other titanias, silica-stabilized titanias, aluminas(including, but not limited to boehmite, gamma and alpha alumina),stabilized aluminas (for example, those stabilized by lanthanum or otherlanthanides), amorphous silicas, silica-aluminas, zeolites (including,but not limited to, faujasite, mordenite, ZSM-5 and Beta zeolite),and/or molecular sieves. In one embodiment, the minority phase of thecapture material used in the selective catalytic reduction catalystsystem of the present invention maintains a total fractional monolayercoverage on the majority phase of about 5 or less under conditionsencountered in accelerated aging tests to simulate the lifetime (e.g.,120,000 miles) of on-road exposure to an SCR catalyst that is positioneddownstream from a DPF. The Ford[1] test conditions are, for example,exposure to 670° C. for 64 hr with a gas-hourly space velocity (GHSV) of30,000 hr⁻¹ with 5 vol % water. Other test conditions which may beemployed are 750° C. for 4 hr and a GHSV of ˜100,000 hr⁻¹ and 5 vol %water. For example, the materials referred to herein as MC-X5v1 andMC-X5v2 have Tf_(m) less than or equal to about 3 after these exposures.In another version illustrative of a material that does not meet therequirements for capture material of the present invention, DT-52 w/2 wt% V₂O₅ has a Tf_(m) greater than about 3 after exposure at therelatively mild condition of 670° C. for 4 hr with GHSV>10,000 hr⁻¹ and˜5% H₂O. Preferably, the exposure conditions simulate the lifetimeexposure of the catalysts under real-world conditions. Also, the degreeof aging that occurs (and hence the final surface area and Tf_(m)) alsodepends on vanadia content. In the present disclosure the vanadiacontent of the catalysts are preferably in a range of between 0.5% and5% vanadia, and more preferably in a range of between 1% and 3% vanadia.

EXAMPLES

The following experimental apparatus was designed to provideillustration of the present invention. SCR catalyst samples consistingof catalyst components that include one or more of vanadia, tungstaand/or silica, were aged at elevated temperatures in a gas stream thatcontains H₂O and O₂, each at 5 vol %, and NO and NH₃, each at 500 ppm,with the balance consisting of N₂. This gas stream is a representativemixture that approximates the composition of gases under realisticconditions. The inorganic vapors generated from the catalyst sampleswere then captured on a “capture bed” located downstream of the catalystbut still in the hot zone of the furnace. For the approach to besuccessful, the volatile component vapors, in this case, the oxides andhydroxides of vanadia and tungsta, must react rapidly and quantitativelywith the capture material at the elevated temperatures of the test. Ifthis condition is met, then the amount captured downstream also is anindirect measure of the vapor pressure oxide of interest. In thefollowing discussion, the two quantities (amount captured and vaporpressure of volatile component) are used interchangeably.

Example 1 Mass Balance

In this example, the use of a catalyst material comprising DT-52 with 2wt % vanadia (prepared by evaporation of an alkalinemonoethanolamine/vanadia solution) was held in a position upstream of acapture bed, in tests to demonstrate that the volatile components fromthe catalyst sample could be quantitatively captured by the downstreambed. The DT-52 support is commercially available from MIC, and has acomposition of 90% TiO₂, 10% WO₃. A gamma alumina (Alfa Aesar, aluminumoxide, gamma, catalyst support, high surface area, bimodal) was used asthe capture bed material and was additionally calcined at 800° C. for 6hr in air, and had a nominal surface area of 200 m²/g. A small quantity,0.2 g of −14/+24 mesh, of the alumina was placed in a reactor tube in aposition downstream of the catalyst sample. The alumina capture bed wasseparated from an equivalent amount (0.2 g) of −14/+24 mesh of thecatalyst sample by a short (<1 cm) plug of quartz wool. A second, shortplug of quartz wool downstream of the alumina capture bed was used tomaintain the position of the alumina. DT-52 with 2 wt % vanadia waschosen as the catalyst sample in this experiment because it is known inthe art that this material does not have a high degree of stability athigh temperatures. To confirm that fact, the surface area of thestarting, vanadia-doped DT-52 was 58 m²/g, while the surface area of theexposed and recovered material (as described below) was 12 m²/g. Hencethe catalyst material underwent significant loss of surface area duringthe exposure. The catalyst material and alumina capture bed materialwere then exposed at 750° C. for 1 hr with a total gas flow of 65 L/hr,and both the catalyst sample and capture bed sample were manuallyrecovered for analysis. The temperature for exposure of 750° C. waschosen instead of 670° C. since the former causes a comparable amount ofcatalyst to occur in a shorter period of time so that the test could beshortened from 64 hr to 1-4 hr, while still providing representativeresults. This gas flow represents a GHSV of ˜200,000 hr⁻¹, and thus ismany-fold higher than that used in the Ford[1] test. However, the higherflow enabled the vapor transport of volatile components to be greatlyaccentuated by virtue of the law of mass action, so as to ease theburden of subsequent volatile component recovery and analysis. Sincelesser amounts of volatile components would be transported and recoveredusing lower flow conditions, the present test is considered to be a verysensitive method for determining the component volatility.

After the exposure, each sample was digested with concentrated aqueousHF, and analyzed by ICPES for tungsten and vanadium. The detectionlimits are 2.5 μg for each V and W per gram (ppm) of capture material(e.g., alumina).

The results, presented as an average of 4 separate runs, are given inTable 1.

TABLE 1 Description V (ppm) W (ppm) Starting Catalyst (DT-52 with 2 wt %vanadia) 10075 72675 Recovered Catalyst 10425 71975 Starting CaptureMaterial (gamma alumina) 0 0 Recovered Capture Material 19 4525 MassBalance. % 104 105 Std. Dev., % 4.5 5.4

It can be seen that the loss of V to the vapor phase is minimal butmeasurable, while the loss of W is appreciable since the recoveredcapture alumina material contained roughly 0.45 wt % W. Also, it can beseen that the mass balances for each are essentially 100%, since themeasured average mass balances are within one standard deviation of thetheoretical value.

Example 2 Demonstration of Stability of Capture Material

This example demonstrates that once the tungsta and vanadia are presenton the surface of the highly stable, high surface area capture material(in this case, gamma alumina) they are not volatile under the testconditions, even at exceedingly high exposure temperatures. Thus, agamma alumina sample (Alfa Aesar, aluminum oxide, gamma, catalystsupport, high surface area, bimodal) was loaded with 47013 ppm W and11200 ppm V (by deposition from alkaline monoethanolamine solution) andsubsequently calcined at 600° C. for 6 hr in air. This catalyst samplewas positioned upstream of a downstream capture material (un-doped gammaalumina) as in Example 1. Separate tests demonstrated that the surfacearea of the W and V-doped alumina was 191 m²/g after exposure at 750° C.for 16 hr in an atmosphere that contained 10% H₂O, which demonstratesthe high stability of the material. The catalyst and capture materialswere then exposed to the reactant stream at 750° C. for 1 hr with a gasflow rate of 65 L/hr, and the spent samples were recovered and analyzed.There was no measurable amount of W or V on the capture bed, so thatthese oxides exhibit no volatility when supported on high surface area,highly stable alumina support. In summary Examples 1 and 2 demonstratetwo important discoveries, namely that both V and W, when supported on atitania of low thermal stability, exhibit measurable volatility at 750°C., but V and W do not exhibit measurable volatility at the sametemperature when supported on high stability, high surface area alumina.

Examples 3 through 7 Evaluation Of Various Vanadia-Based Of CatalystMaterials

The discoveries described in Examples 1 and 2 presented a method of thepresent invention as a means to investigate the volatility of variouscatalyst components such as V and W at the laboratory scale. Thus, inthe following examples, SCR catalysts that contain tungsta and vanadiasupported on various titania-based supports, were screened for thevolatility of the catalyst components. The materials in these Examplesall contained 2 wt % V₂O₅, deposited from alkaline monoethanolaminesolution, and the V-doped materials were calcined at 600° C. for 6 hr inair to remove the water and organic components. The DT-58 base materialis a commercially available titania-based SCR catalyst support availablefrom MIC. The composition of the DT-58 support is 81% TiO₂, 9% WO₃ and10% SiO₂. The samples in Examples 5 and 6, labeled MC-X5v1 and MC-X5v2,are developmental SCR catalyst supports as described hereinbelow inreference to “Stabilized Anatase Titania” and in U.S. patent applicationSer. No. 12/533,414 which is expressly incorporated herein by referencein its entirety. The composition of the MC-X5v1 support is 90% TiO₂, 4%SiO₂ and 6% WO₃, while the composition of the MC-X5v2 support is 88%TiO₂, 8% SiO₂ and 4% WO₃.

TABLE 2 Surface Pore Material Area Volume V W Ex. [a] Condition (m²/g)(cm³/g) (ppm) (ppm) TFm 4a DT-58 670 C., 64 h, 10% H₂O, 6 L/h 55.7 0.25N/A N/A N/A 4 DT-58 (b) 750 C., 4 h, 5% H₂O, 32.5 L/h 43.0 0.28 42.57150 5.6 5 MC-X5v1 (b) 750 C., 4 h, 5% H₂O, 32.5 L/h 38.7 0.25 1.7 753.53.1 3 DT-52 (b) 750 C., 4 h, 5% H₂O, 32.5 L/h 8.3 0.05 82.6 8,050 9.3 6MC-X5v2 (b) 750 C., 4 h, 5% H₂O, 32.5 L/h 59.6 0.29 0.0 190.5 3.0 [a] =2 wt % V₂O₅ (b) = average of multiple runs

It was first desired to determine a set of conditions that would berepresentative of the real-life exposure of a catalyst. As describedabove, Ford researchers[1] have developed an accelerated aging protocolfor SCR catalysts that are positioned downstream of a diesel particulatefilter (DPF), that simulates on-road exposure for 120,000 miles. Theaccelerated aging test involves exposing the catalyst to a reactant gasstream that includes water (5 vol %), for a time of 64 hr at 670° C. atGHSV=30,000 hr⁻¹. These exposure conditions represent the extremelyharsh conditions that would occur as the result of the high temperaturescreated during soot-combustion during regeneration of the DPF, and arenot normally encountered in traditional SCR applications.

Thus, the DT-58 and MC-X5v1 catalysts were exposed for 64 hr at 670° C.in an atmosphere that contained 10 vol % H₂O, and the surface area andpore volumes of the aged catalysts was determined as shown in Table 2.The same starting catalyst materials were also utilized as the catalystsamples in a dual bed configuration upstream of a downstream gammaalumina capture bed, and were treated at 750° C. as described inExamples 1 and 2, above, only the exposure time was 4 hr and thereactant gas flow was 32.5 L/hr (equivalent to GHSV˜100,000 hr⁻¹). Thegas flow rate under these conditions is still higher than in the Ford[1]test, but is more closely representative of that test.

The results in Table 2 show that the surface areas for the catalystsexposed in the test method of the present invention at 750° C. for 4 hrwere slightly lower than for the same materials aged at 670° C. for 64hr. Thus, if the surface area is used as a measure of the extent towhich the catalyst samples have aged (and hence severity), then theconditions in the former test are slightly more severe than in theFord[1] test. Thus, it is concluded that the test of the presentinvention, when conducted at 750° C. for 4 hr with a reactant gasflowing at 32.5 L/hr, is a good first approximation to the real-lifeexposure of the catalyst over the duration of 120,000 miles of on-roaduse.

Each of the catalysts listed in Table 2 were evaluated multiple timesand average results are shown therein. The results in Table 2demonstrate that the catalysts show varying degrees of loss of V and W.Also shown in Table 2 are the Total Fractional Monolayer (Tf_(m)) valuesfor each material, where the surface area is that of the aged samples.Shown in FIGS. 3 and 4 are the correlations found between the amount ofV and W lost from the catalyst samples and the Tf_(m) values. The dataand graphs show that when the TF_(m) values are equal to or less thanabout 3.1, the vanadia and tungsta catalyst components exhibit lowvolatility and are substantially retained on the catalyst material,while when the TF_(m) values are greater than 3.1, the catalystcomponents exhibit much higher volatility and are lost from the catalystmaterial (but are captured on the capture material). Of course, evenwith the relatively high levels of V and W that are lost from the DT-52catalyst material, when these volatile oxides and hydroxides arecaptured on the capture bed material, e.g., alumina, the TF_(m) for thatmaterial is much less than 1 after the exposure. These good correlationscan be used to predict the behavior of unknown materials that might beuseful as catalyst and capture materials. Thus, vanadia-based titaniamaterials that have high aged surface areas and low amounts of addedcatalyst components such as SiO₂ and WO₃ will demonstrate little to noloss of W and V to the vapor phase under these harsh exposureconditions, and hence are attractive catalyst materials. Such supportmaterials will also be good capture materials in the event that lessstable, high TF_(m) catalyst materials such as vanadia on DT-52 arepositioned upstream or in intimate mixture with the more stable, lowTF_(m) materials.

Without being bound by theory, it is believed that the reason for thelow volatility of the minority-phase components on alumina or stabilizedtitania (or other materials contemplated herein) is that when theminority-phase components are present at low fractional coverage (lowf_(m)), they interact strongly chemically with the majority-phasesupport, and this favorable interaction energy in turn lowers theequilibrium constants involved in the vaporization of the supportedcomponents.

Stabilized Anatase Titania.

In preferred embodiments, the material which comprises the majorityphase catalyst material and/or the capture bed material used herein isan anatase titania (described in more detail in U.S. Ser. No.12/533,414), wherein the anatase titania is stabilized by a silicaprovided in a low molecular weight form and/or small nanoparticle form.Further, the minority phase disposed on the anatase titania preferablycomprises vanadia (and optionally tungsta), for vanadia-based selectivecatalytic reduction of DeNOx from lean-burn (diesel) engines.

The actual specific composition of the silica-titania orsilica-titania-tungsta catalyst support material (majority phase) and/orcapture bed material may be dictated by the requirements of the specificcatalytic application. In one preferred composition, the materialcomprises a silica-stabilized titania material which comprises particleswhich contain ≧90% dry weight of TiO₂ and ≦10 wt % SiO₂. In anotherpreferred composition, the material comprises a silica stabilizedtitania-tungsta material with ≧85% dry weight titania, 3%-10% dry weightof SiO₂, and 3%-10% dry weight of WO₃. Alternatively, in one embodimentwhere the application requires particularly good thermal stability, thematerial comprises ≧85% dry weight of TiO₂, 5.0%-9.0% dry weight ofSiO₂, and 3.0%-7.0% dry weight of WO₃. More particularly, the materialmay comprise 87%-89% dry weight of TiO₃, 7%-9% dry weight of SiO₂, and3%-5% dry weight of WO₃. In one preferred embodiment the materialcomprises about 88% (±0.5%) dry weight TiO₂, about 8% (±0.5%) dry weightSiO₂, and about 4% (±0.5%) dry weight WO₃. In one embodiment, the weight% of WO₃ is less than the weight of % of SiO₂. In one embodiment, thematerial has a fresh surface area of at least 80 m²/gm, and morepreferably at least 100 m²/gm.

In another embodiment where the application requires particularly goodcatalytic activity or capture of volatiles, the material comprises ≧85%dry weight of TiO₂, 3.0%-8.0% dry weight of SiO₂, and 4.0%-9.0% dryweight of WO₃. More particularly, this active material comprises ≧87%dry weight of TiO₃, 3%-6% dry weight of SiO₂, and 4%-8% dry weight ofWO₃. In one preferred embodiment the material comprises about 90%(±0.5%) dry weight TiO₂, about 4% (±0.5%) dry weight SiO₂, and about 6%(±0.5%) dry weight WO₃. In one embodiment, the weight % of WO₃ isgreater than the weight of % of SiO₂.

In an embodiment of the invention, the TiO₂ component of the materialused herein substantially comprises a surface area <400 m²/g and a porevolume <0.40 cm³/g.

In one embodiment, the material is produced by mixing a titania slurryand a silica component at a temperature <80° C. and at a pH <8.5.Alternatively, the titania slurry and silica component used herein maybe mixed at a temperature <70° C. and at a pH <7.0.

The vanadia catalyst of the invention may comprise the silica-stabilizedtitania or titania-tungsta catalyst support described herein upon whicha quantity of vanadium oxide (V₂O₅) is disposed, wherein the V₂O₅preferably comprises 0.5% to 1% to 2% to 3% to 4% to 5% of the dryweight thereof. The vanadia catalyst materials of the invention may befurther treated by calcination (sintering) at a temperature ≧650° C. toincrease their deNOx catalytic activity.

The emission system of the invention with may be used with a dieselparticulate filter (DPF) upstream of the engine or downstream of theengine.

Preferably most of the silica particles in the silica-stabilized titaniamaterial have diameters <5 nm, and more preferably <4 nm and morepreferably <3 nm, and still more preferably <2 nm, and/or comprise lowmolecular weights (e.g., MW<100,000), whether or not the particles do,or do not, have V₂O₅ deposited thereon.

Where the silica titania material also has V₂O₅ deposited thereon theV₂O₅ preferably comprise from 0.5% to 3.0% to 5% of the dry weight ofthe material.

Distribution of the WO₃ and SiO₂ species on the surface of the titaniamaterial also plays a role in the optimization of DeNOx activity of thevanadia catalysts. Thus, when the catalysts are freshly prepared, thatis, when the added silica and tungsta oxides are first deposited andbefore high temperature treatment, the fractional monolayer coverage ispreferably about 1.0 or less.

The SiO₂ may be present at a fractional monolayer value of less than 1.0before the material is sintered. The small nanoparticle form of the SiO₂may comprise a diameter of <5 nm. The low molecular weight form of theSiO₂ may comprise a MW of <100,000. The SiO₂ may comprise silicon atomswhich are substantially (e.g., >50%) in the Q³, Q², Q¹ and Q⁰coordination environments. The SiO₂ may comprise patches which aresubstantially ≧5 nm deep after redistribution as seen by scanningelectron microscopy or by transmission electron microscopy. The TiO₂used may optionally not be prepared in the presence of urea.

In another aspect, the invention may be a vanadia catalyst comprising asilica-stabilized titania material as described herein which comprisesV₂O₅ disposed thereon. The vanadia catalyst may comprise, for example,0.5% to 5% dry weight of V₂O₅ (or more preferably 1.0 to 3%). The V₂O₅may be present at a fractional monolayer value of less than 1.0 beforesintering. The vanadia catalyst may be sintered at ≧650° C. for example.In another aspect, the invention may be a diesel selective catalyticreduction catalyst system comprising the vanadia catalyst and capturebed material as described herein. In another aspect a diesel engineexhaust treatment system may further comprise a diesel particulatefilter, and wherein the present catalytic capture bed device ispositioned upstream of or downstream of the diesel particulate filter.

In another one of its aspects, the invention is a method of catalyzingthe conversion of nitrogen oxides to N₂ gas, comprising exposing engineemissions comprising NOx to the vanadia catalyst as described hereinwith an added reductant to produce N₂ and H₂O. The reductant may be forexample NH₃ and/or urea. In the method the vanadia catalyst may comprise0.5%-5% (or more preferably 1.0% to 3%) dry weight of V₂O₅, for example.The engine emissions may be passed through a diesel particulate filterbefore or after being exposed to the vanadia catalyst wherein theemissions are then passed through the capture bed material.

As noted above, the stabilization of titania material with silicapreferably involves treatment of the titania with silica in a lowmolecular weight form and/or small nanoparticle form, such astetra(alkyl)ammonium silicate (e.g., tetramethylammonium silicate) ortetraethylorthosilicate (TEOS). Other examples of low molecular weightand/or small nanoparticle silica precursors which may be used in thepresent invention include, but are not limited to aqueous solutions ofsilicon halides (i.e., anhydrous SiX₄, where X=F, Cl, Br, or I), siliconalkoxides (i.e., Si(OR)₄, where R=methyl, ethyl, isopropyl, propyl,butyl, iso-butyl, see-butyl, tert-butyl, pentyls, hexyls, octyls,nonyls, decyls, undecyls, and dodecyls, for example), othersilicon-organic compounds such as hexamethyldisilazane, fluoro-silicicacid salts such as ammonium hexafluorosilicate [(NH₄)₂SiF₆], quaternaryammonium silicate solutions (e.g., (NR₄)n, (SiO₂), where R=H, or alkylssuch as listed above, and when n=0.1-2, for example), aqueous sodium andpotassium silicate solutions (Na₂SiO₃, K₂SiO₃, and MSiO₃ wherein M is Naor K in varying amounts in ratio to Si), silicic acid (Si(OH)₄)generated by ion exchange of any of the cationic forms of silica listedherein using an acidic ion-exchange resin (e.g., ion-exchange of thealkali-silicate solutions or quaternary ammonium silicate solutions). Inpreferred embodiments, the titania used herein has not been prepared inthe presence of urea.

The catalyst support material and/or capture bed material may beproduced by providing a slurry comprising TiO₂, combining the TiO₂slurry with (1) a silica precursor solution comprising SiO₂substantially in a low molecular weight form and/or SiO₂ comprisingsmall nanoparticles and with (2) WO₃ to form a TiO₂—WO₃—SiO₂ mixture,wherein the silica precursor solution is combined with the TiO₂ slurrybefore, after, or while the WO₃ is combined with the TiO₂ slurry, andthen washing and sintering the TiO₂—WO₃—SiO₂ mixture to form asilica-stabilized titania support material. In the method thesilica-stabilized titania support material may comprise, for example,86%-94% dry weight of TiO₂, 3%-9% dry weight of a SiO₂, and 3%-7% dryweight of WO₃, and the titania support material may primarily comprise asurface area of at least 80 m²/gm before sintering. The TiO₂ of theslurry may comprise, for example, preformed titanium hydroxide, titaniumoxy-hydroxide or titanium dioxide particles. Optionally, the TiO₂ of theslurry is not produced in the presence of urea.

The silica precursor solution may comprise a tetra(alkyl)ammoniumsilicate solution or silicic acid. The SiO₂ may substantially comprisepatches which are ≦5 nm in depth after redistribution as seen byscanning electron microscopy or by transmission electron microscopy. Themethod may further comprise combining the TiO₂—WO₃—SiO₂ mixture withV₂O₅ to form a vanadia catalyst. The vanadia catalyst thus formed maycomprise, for example, 0.5% to 3% to 5% dry weight of V₂O₅. The V₂O₅thereof may be present at a fractional monolayer value of less than 1.0before sintering. The vanadia catalyst may be sintered at ≧650° C. forexample.

Alternatively, the silica-stabilized titania material may be produced byproviding a TiO₂ slurry comprising TiO₂ particles, providing aparticulate silica source, combining the TiO₂ slurry with theparticulate silica source to form a TiO₂—SiO₂ mixture, and adjusting theTiO₂—SiO₂ mixture to a pH <8.5 and a temperature <80° C. wherein theparticulate silica source is dissolved and reprecipitated on the TiO₂particles to form the silica-stabilized titania material. The method mayfurther comprise the step of combining the silica-stabilized titaniamaterial with WO₃ to form a silica-stabilized titania tungsten material.The method may further comprise washing and sintering thesilica-stabilized titania tungsten material. The silica-stabilizedtitania tungsten material may comprise, for example, 86%-94% dry weightof TiO₂, 3%-9% dry weight of a SiO₂, and 3%-7% dry weight of WO₃, andthe titania material may primarily comprise a surface area of at least80 m²/gm before sintering. The TiO₂ particles of the TiO₂ slurry maycomprise, for example, preformed titanium hydroxide, titaniumoxy-hydroxide or titanium dioxide particles. The SiO₂ of the TiO₂—SiO₂mixture, after dissolving, may comprise silicon atoms which aresubstantially (e.g., >50%) in the Q³, Q², Q¹ and Q⁰ coordinationenvironments. The SiO₂ on the TiO₂ particles of the method maysubstantially comprise patches which are ≦5 nm in depth afterredistribution of the SiO₂ as seen by scanning electron microscopy or bytransmission electron microscopy. The method may further comprisecombining the TiO₂—WO₃—SiO₂ mixture with V₂O₅ to form a vanadiacatalyst. In the method, the vanadia catalyst may comprise, for example,0.5%-3% dry weight of V₂O₅. The V₂O₅ of the vanadia catalyst may bepresent at a fractional monolayer value of less than 1.0 beforesintering, and the vanadia catalyst may be sintered at ≧650° C.

As contemplated herein, in one embodiment, the invention is a selectivecatalytic reduction catalyst system for treating diesel exhaust gascontaining nitrogen oxides and diesel soot particulates. The systemcomprises a catalyst material and a capture material. The catalystmaterial comprises a majority phase which may comprise a titania-basedsupport material, and a minority phase comprising a catalyst componentcomprising at least one oxide of vanadium, silicon, tungsten,molybdenum, iron, cerium, phosphorous, copper or manganese. The capturematerial comprises a majority phase for capturing a minority phasecomprising volatile oxides or hydroxides originating from the catalystmaterial, wherein the minority phase of the capture material maintains atotal fractional monolayer coverage on the majority phase of the capturematerial of about 5 or less. The capture material is positioned in amixture with the catalyst material, or is located downstream of thecatalyst material or is positioned in a mixture with the catalystmaterial and is located downstream of the catalyst material. Theminority phase of the capture material in this embodiment may maintain atotal fractional monolayer coverage of 5 or less on the majority phaseof the capture material when exposed to conditions of 750° C. for 4hours at a gas-hourly space velocity of 100,000 hr⁻¹ and 5 vol % water.The minority phase catalyst components of the catalyst material maymaintain a total fractional monolayer value of 5 or less on the majorityphase after exposure to conditions of 750° C. for 4 hours at agas-hourly space velocity of 100,000 hr⁻¹ and 5 vol % water. The capturematerial is preferably able to remove substantially all volatile oxidesand hydroxides originating from the catalyst material. The majorityphase of the capture material may primarily comprise at least one ofaluminas, stabilized aluminas, silicas, silica-aluminas, amorphoussilicas, titanias, silica-stabilized titanias, zeolites or molecularsieves or combinations thereof. Where the majority phase is a stabilizedalumina, the stabilized aluminas may be stabilized by lanthanum or otherlanthanides. The majority phase of the capture material and the majorityphase of the catalyst material may comprise titania stabilized withsilica.

In another embodiment the present invention contemplates a diesel engineexhaust treatment system comprising a selective catalytic reductioncatalyst system and a diesel particulate filter for treating dieselexhaust gas containing nitrogen oxides and diesel soot particulates. Thecatalyst system comprises a catalyst material and a capture material.The catalyst material comprises a majority phase which may comprise atitania-based support material, and a minority phase comprising acatalyst component comprising at least one oxide of vanadium, silicon,tungsten, molybdenum, iron, cerium, phosphorous, copper or manganese.The capture material may comprise a majority phase for capturing aminority phase comprising volatile oxides or hydroxides originating fromthe catalyst material, wherein the minority phase of the capturematerial maintains a total fractional monolayer coverage on the majorityphase of the capture material of about 5 or less. The capture materialmay be positioned in a mixture with the catalyst material, or may belocated downstream of the catalyst material, or may be positioned in amixture with the catalyst material and located downstream of thecatalyst material. The selective catalytic reduction catalyst system maybe positioned upstream of or downstream of the diesel particulatefilter. The minority phase of the capture material in this embodimentmay maintain a total fractional monolayer coverage of 5 or less on themajority phase of the capture material when exposed to conditions of750° C. for 4 hours at a gas-hourly space velocity of 100,000 hr⁻¹ and 5vol % water. The minority phase catalyst components of the catalystmaterial may maintain a total fractional monolayer value of 5 or less onthe majority phase after exposure to conditions of 750° C. for 4 hoursat a gas-hourly space velocity of 100,000 hr⁻¹ and 5 vol % water. Thecapture material is preferably able to remove substantially all volatileoxides and hydroxides originating from the catalyst material. Themajority phase of the capture material may primarily comprise at leastone of aluminas, stabilized aluminas, silicas, silica-aluminas,amorphous silicas, titanias, silica-stabilized titanias, zeolites ormolecular sieves or combinations thereof. Where the majority phase is astabilized alumina, the stabilized aluminas may be stabilized bylanthanum or other lanthanides. The majority phase of the capturematerial and the majority phase of the catalyst material may comprisetitania stabilized with silica.

In another embodiment, the present invention is a method of treatingdiesel exhaust gas comprising the steps of providing a selectivecatalytic reduction catalyst system, with or without a dieselparticulate filter, and exposing the diesel exhaust gas to the selectivecatalytic reduction catalyst system wherein the capture material removessubstantially all volatile oxides and hydroxides originating from thecatalyst material from the diesel exhaust gas. In this method theselective catalytic reduction catalyst system comprises a catalystmaterial and a capture material. The catalyst material comprises amajority phase which may comprise a titania-based support material, anda minority phase comprising a catalyst component comprising at least oneoxide of vanadium, silicon, tungsten, molybdenum, iron, cerium,phosphorous, copper or manganese. The capture material comprises amajority phase for capturing a minority phase comprising volatile oxidesor hydroxides originating from the catalyst material, wherein theminority phase of the capture material maintains a total fractionalmonolayer coverage on the majority phase of the capture material ofabout 5 or less. The capture material is positioned in a mixture withthe catalyst material, or is located downstream of the catalyst materialor is positioned in a mixture with the catalyst material and is locateddownstream of the catalyst material. The minority phase of the capturematerial in this embodiment may maintain a total fractional monolayercoverage of 5 or less on the majority phase of the capture material whenexposed to conditions of 750° C. for 4 hours at a gas-hourly spacevelocity of 100,000 hr⁻¹ and 5 vol % water. The minority phase catalystcomponents of the catalyst material may maintain a total fractionalmonolayer value of 5 or less on the majority phase after exposure toconditions of 750° C. for 4 hours at a gas-hourly space velocity of100,000 hr⁻¹ and 5 vol % water. The capture material is preferably ableto remove substantially all volatile oxides and hydroxides originatingfrom the catalyst material. The majority phase of the capture materialmay primarily comprise at least one of aluminas, stabilized aluminas,silicas, silica-aluminas, amorphous silicas, titanias, silica-stabilizedtitanias, zeolites or molecular sieves or combinations thereof. Wherethe majority phase is a stabilized alumina, the stabilized aluminas maybe stabilized by lanthanum or other lanthanides. The majority phase ofthe capture material and the majority phase of the catalyst material maycomprise titania stabilized with silica.

While the invention has been described in connection with certainpreferred embodiments and examples herein so that aspects thereof may bemore fully understood and appreciated, it is not intended to limit theinvention to these particular embodiments and examples. Thus, thepresent examples, which include preferred embodiments serve toillustrate the practice of this invention, it being understood that theparticulars shown are by way of example and for purposes of illustrativediscussion of preferred embodiments of the present invention only andare presented in the cause of providing what is believed to be the mostuseful and readily understood description of formulation procedures aswell as of the principles and conceptual aspects of the invention.

Therefore, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the machines, processes, itemsof manufacture, compositions of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,machines, processes, items of manufacture, compositions of matter,means, methods, or steps, presently existing or later to be developedthat perform substantially the same function or achieve substantiallythe same result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such machines,processes, items of manufacture, compositions of matter, means, methods,or steps.

Each of the patents, published patent applications, references andarticles cited herein are hereby expressly incorporated herein byreference in their entireties.

CITED REFERENCES

1. G. Cavataio, et al., Society of Automotive Engineers 2007-01-1575,455 (2007).

2. Weckhuysen, B. M., and Keller, D. E., “Chemistry, Spectroscopy andthe Role of Supported Vanadium Oxides in Heterogeneous Catalysis”,Catalysis Today 78:25-46, 2003.

3. Wachs, I. E., Kim, T., Ross, E. I., “Catalysis Science of the solidAcidity of Model Supported Tungsten Oxide Catalysts”, Catalysis Today116:162-168, 2006.

4. Iler, R. K. The Chemistry of Silica: Solubility, Polymerization,Colloid and Surface Properties, and Biochemistry, John Wiley & Sons, NewYork, ISBN 0-471-02404-x:31, 1979.

What is claimed is:
 1. A method of treating diesel exhaust gas,comprising: exposing diesel exhaust gas to a selective catalyticreduction catalyst system comprising: a catalyst bed materialcomprising: a majority phase containing a titania-based supportmaterial, and a minority phase containing a catalyst component whichincludes vanadium oxide; and a capture bed material containing amajority phase for capturing a minority phase comprising volatile metaloxides or volatile metal hydroxides originating from the catalyst bedmaterial, wherein the minority phase of the capture bed material ismaintained at a total fractional monolayer coverage on the majorityphase of the capture bed material of 3.1 or less; and wherein thecapture bed material removes substantially all volatile metal oxides andvolatile metal hydroxides originating from the catalyst bed materialfrom the diesel exhaust gas.
 2. The method of claim 1 wherein theminority phase of the capture bed material of the selective catalyticreduction catalyst system is maintained at a total fractional monolayercoverage on the majority phase of the capture bed material of 3.1 orless when exposed to conditions of 750° C. for 4 hours at a gas-hourlyspace velocity of 100,000 hr⁻¹ and 5 vol % water.
 3. The method of claim1 wherein the minority phase catalyst components of the catalyst bedmaterial of the selective catalytic reduction catalyst system aremaintained at a total fractional monolayer value of 5 or less on themajority phase after exposure to conditions of 750° C. for 4 hours at agas-hourly space velocity of 100,000 hr⁻¹ and 5 vol % water.
 4. Themethod of claim 1 wherein the majority phase of the capture bed materialof the selective catalytic reduction catalyst system primarily comprisesat least one of aluminas, stabilized aluminas, silicas, silica-aluminas,amorphous silicas, titanias, silica-stabilized titanias, zeolites ormolecular sieves or combinations thereof.
 5. The method of claim 4wherein the stabilized aluminas of the capture bed materials arestabilized by lanthanum or other lanthanides.
 6. The method of claim 1wherein the majority phase of the capture bed material and the majorityphase of the catalyst bed material of the selective catalytic reductioncatalyst system comprise titania stabilized with silica.
 7. The methodof claim 1 wherein the selective catalytic reduction catalyst system isprovided as a component of a diesel engine exhaust treatment systemwhich further comprises a diesel particulate filter.
 8. The method ofclaim 1 wherein the minority phase of the catalyst bed material furthercontains at least one oxide of silicon, tungsten, molybdenum, iron,cerium, phosphorus, copper, manganese, or a combination thereof.